docs: refresh for OpenCode v1.16.2 and fix correctness bugs

[wesammustafa]

Update the guide to the current release (v1.16.2, 2026-06-05), verified against opencode.ai/docs and github.com/anomalyco/opencode.

Correctness fixes:

• Plugins: tool.execute.before/after and shell.env are top-level hooks with an (input, output) signature, not event.type values. Rewrite both shipped example plugins plus the docs/migration examples so they fire.
• Permissions: bash glob maps resolve last-matching-rule-wins (catch-all "" first), not most-specific-wins. Fix prose, reorder examples, and fix the shipped opencode.json (rm -rf deny was shadowed by a later rm *).
• Drop non-existent permission keys repo_clone/repo_overview (15 keys, not 17); fix steps (integer), deprecate maxSteps/tools, move reasoningEffort into model options; fix the mise install command.

Currency updates:

• v1.15.4 -> v1.16.2; add v1.16 highlights and a changelog section.
• Refresh OpenCode Zen lineup/pricing (Claude Opus 4.8, gpt-5.4, gemini-3.5-flash, qwen3.7, grok-build-0.1; correct free-tier IDs).
• Add opencode mcp add, run --replay, extra skill paths, shift+tab, and native GitHub commands; correct the specialist-prompt count; bump dates.

What this changes

Type

• Fact correction (something here is out of date or wrong)
• New content (doc, MCP walkthrough, specialized-agent prompt, etc.)
• Restructure / cleanup
• Typo / formatting / lint fix

Sources

Checklist

• I read CONTRIBUTING.md
• Every factual claim cites a source (in the PR or inline in the doc)
• In-page anchors and cross-doc links still resolve
• No trailing whitespace; code fences have language tags
• Emoji follow the AGENTS.md policy (H2 navigation + callout prefixes only)
• If I touched the README, I updated the "Last reviewed" line at the bottom